1. Field of the Invention
The present invention relates to a planar light source for illuminating a liquid crystal panel and the like from their back.
2. Description of the Prior Art
Small liquid crystal displays have been used in recent years in cellular phones and other portable information terminals and, as a means for illuminating such liquid crystal displays, a planar light source is used. The planar light source is constructed of a plate-like light guide plate and light emitting diodes (LEDs) arranged to face a side surface of the light guide plate. Thanks to its ability to be reduced easily in size and thickness, the planar light source has found wide uses.
In the planar light source, light emitted from the LEDs enters into the light guide plate and propagates as it is repetitively reflected within the light guide plate. The light is reflected or refracted by grooves or a textured pattern formed in an underside of the light guide plate until it goes out of the plate. The light emitted from the top surface of the light guide plate travels toward and illuminates the liquid crystal display.
An example of such a conventional planar light source is shown in FIG. 6 (e.g., Japanese Patent Application No. 2002-093383, FIG. 7). FIG. 6 illustrates a construction of a planar light source 110 which has LEDs 101 as a light source, a light guide plate 102, a prism sheet 103, a reflector plate 106 and a liquid crystal panel 107. The light guide plate 102 is rectangular and made of a light-transmitting glass or resin. Denoted 102a is a top surface of the light guide plate 102. Designated 102c is a light receiving side surface facing the LEDs 101. Designated 102b is a bottom surface of the light guide plate 102. The bottom surface 102b is formed with a plurality of asymmetric prisms 102b1 facing the top surface 102a. The asymmetric prisms 102b1 each comprise a down slope 102b11 whose distance to the top surface 102a sharply increases as it moves away from the light receiving side surface 102c and an up slope 102b12 whose distance to the top surface 102a moderately decreases. Arranged opposite the light receiving side surface 102c are three LEDs 101 supported on a retainer member 101b. 
When a predetermined amount of electricity is supplied from a power supply not shown to the LEDs 101, the LEDs 101 illuminate in white or a predetermined color. The light emitted from the LEDs 101 is refracted by the light receiving side surface 102c as it enters the light guide plate 102. The light that has entered the light guide plate 102 is repetitively reflected between the top surface 102a and the bottom surface 102b of the light guide plate 102 before it is refracted by the top surface 102a and leaves the light guide plate 102. The light then enters the prism sheet 103, in which it is specular-reflected until its propagation direction is changed to a Z direction. The light traveling in the Z direction is now incident on the liquid crystal panel 107. Therefore, the light passes through the liquid crystal in an ideal direction, making a clear and vivid image display possible.
FIG. 7 is a side view showing a path of light emitted from the LEDs 101 that has entered the light guide plate 102. In the figure, a light ray emitted from the LEDs 101 at an output or emittance angle of θi enters the light receiving side surface 102c of the light guide plate 102 at an incidence angle of θi. At this time, the ray is refracted on this plane and a relationship between the incidence angle θi and a refracted angle θ is, according to Snell's law, n·sin θ=sin θi assuming that a refractive index of air is 1 and a refractive index of the light guide plate 102 (made of polycarbonate and the like) is n. From this we obtainθ=sin−1((1/n)sin θi)  (1)If, for example, the light guide plate 102 has a refractive index of n=1.58 and θi=90°, calculating the equation (1) results in θ=sin−1(1/1.58)=39.3° and thus the critical angle θc is θc=39.3°.
It should be noted, however, that since the incidence angle in reality is less than 90° at maximum, the refracted angle θ even at its maximum is less than the critical angle θc. The critical angle θc of the light guide plate 102 is generally around 40°, so the refracted angle θ even at its maximum does not exceed 40°. The light ray that has passed through the light receiving side surface 102c at the refracted angle θ is incident on the top surface 102a of the light guide plate 102 at an incidence angle θ1. At this time, as can be seen from FIG. 7, since a relation of (θ+θ1=90°) holds and the refracted angle θ is equal to or less than 40°, as described above, the incidence angle θ1 is equal to or more than 50°, which is larger than the critical angle θc of around 40°. Thus, the ray incident on the top surface 102a is totally reflected at a reflection angle θ1.
The reflected light then strikes, at an incidence angle of θ2=θ1−α, the up slope 102b12 of the bottom surface which has an inclination angle of α. Here the inclination angle α is about 1° to several degrees.
The ray that has struck the up slope 102b12 at an incidence angle θ2 is reflected by this surface at a reflection angle θ2 and then strikes the top surface 102a at an incidence angle of θ3=θ2−α=θ1−2α. The ray is then reflected by the top surface 102a at a reflection angle θ3 to hit the up slope 102b12 at an incidence angle of θ4=θ3−α=θ1−3α. Each time the light ray, that was first reflected by the top surface 102a at a reflection angle θ1, strikes the up slope 102b12 or the top surface 102a, its incidence angle decreases by an amount equal to the inclination angle α. That is, when the ray, that was first reflected at a reflection angle of θ1, strikes the up slope 102b12 or top surface 102a for an Nth time after repetitive reflections, its incidence angle θN is given byθN=θ1−(N−1)α  (2)In this light guide plate, the light incidence or reflection on its boundary surface, shown at θ1, is counted as the first incidence/reflection (i.e., N=1).
When the decreasing incidence angle θN has the following relation with the critical angle θc:θN=θ1−(N−1)α<θc  (3)then, the ray passes through the top surface 102a or the up slope 102b12 of the bottom surface 102b and gets out of the light guide plate 102. For example, if θ1=52°, α=1° and θc=40°, the condition of equation (3) is met when N is more than 13. This means that the light ray must strike the top or bottom surface of the light guide plate 102 fourteen times or more. Therefore, near the light receiving side surface 102c the ray does not escape to the outside. For example, if the light guide plate 102 has a thickness of 1 mm, the ray does not exit the light guide plate 102 from within about 3 mm of the light receiving side surface 102c but normally exits from a region more than 3 mm away from the light receiving side surface 102c. In the region from which the light ray exits normally, there are no light intensity variations.
However, conventional backlight units using such a light guide plate 102 often have the following problems. As shown in a plan view of FIG. 8, in an area S1 within 3–4 mm of the light receiving side surface 102c of the light guide plate 102 several bright lines show up (in FIG. 8 the bright lines are shown hatched with thick lines). S2 represents an area where bright lines do not show. The conspicuous bright lines 14 are considered to be caused as follows. As shown in FIG. 10, rays of light emitted from the LEDs 101 enter the light guide plate 102 from an edge portion 102d where the light receiving side surface 102c and the top surface 102a of the light guide plate 102 cross each other.
In FIG. 11 representing a “light source directivity of LED,” those of the rays emitted from LEDs 101 which reach the edge portion 102d are light traveling in a direction that corresponds to a region SL shown hatched.
If the edge portion 102d has a rough surface, rather than a mirror surface, the light rays from the LEDs 101 enter the edge portion 102d not through normal refraction but through scattering. That is, from the edge portion 102d a plurality of rays travel through the light guide plate 102 in different directions, making the edge portion 102d look as if it were illuminating. Thus, the edge portion 102d can be regarded as a secondary light source. A similar secondary light source also occurs at a lower edge portion 102e of the light guide plate 102 where the light receiving side surface 102c and the bottom surface 102b intersect. In the conventional light guide plate 102 the edge portions 102d, 102e are formed almost at right angles, so their transferability in a molding process is bad, making their surface rough, which in turn results in secondary light sources being easily formed.
As to the secondary light source illuminating at the edge portion 102d, an incidence angle θb that the light from the secondary light source has with respect to the up slope 102b12 of the bottom surface 102b can be smaller than the critical angle θc, as shown in FIG. 9. In that case, as illustrated by a ray s21, the light ray passes through the up slope 102b12 and is then reflected by the reflector plate 106 under the slope to reenter the light guide plate 102 and pass through the top surface 102a, illuminating the light guide plate 102 upward. Because the range of this light emission is wide, no bright lines are produced.
In a case where the incidence angle θb is larger than the critical angle θc, the ray is reflected by the up slope 102b12 and exits through the top surface 102a, as indicated by a ray s22. The number of times that the ray strikes, as by reflection, the top or bottom surface before it exits, the light guide plate 102 varies depending on a difference between the incidence angle θb and the critical angle θc, as explained above. As the difference increases, so does the number of times the ray needs to strike the top or bottom surface. This is described in more detail by referring to FIG. 10. In FIG. 10, φ1, φ2, φ3 and φ4 represent light fluxes emanating from the edge portion 102d and an angular range of each flux is assumed to be smaller than the inclination angle α of the up slope 102b12. Concerning refracted angles of these light fluxes on the top surface 102a as they exit the light guide plate 102, it is assumed that the refracted angle of the light flux φ1 is the smallest, with those of φ2, φ3 and φ4 progressively increasing in that order in steps of the inclination angle α.
Let the refracted angles of the light fluxes φ1, φ2, φ3 and φ4 be θd1, θd2, θd3 and θd4 respectively. These refracted angles are assumed to have the following relations with the critical angle θc:θd1=1.5α+θcθd2=2.5α+θcθd3=3.5α+θcθd4=4.5α+θc  (4)
As shown in FIG. 10, all of the light fluxes φ1, φ2, φ3, φ4 strike for a first time the boundary surface, or the up slope 102b12, and their incidence angles at this time areθd1−α, θd2−α, θd3−α and θd4−αrespectively. From equation (4), all these incidence angles are larger than the critical angle θc and therefore reflected by the up slope 102b12. The reflected light fluxes strike for a second time the boundary surface, or the top surface 102a, and their incidence angles at this time are:θd1−2α<θc, θd2−2α>θc, θd3−2α>θc and θd4−α>θc.Only the incidence angle of the light flux φ1, θd1−2α, is less than the critical angle θc and this flux φ1 with a width b1 exits from the top surface 102a. 
The remaining fluxes φ2, φ3, φ4 are reflected by the top surface 102a and are incident for a third time on the boundary surface, this time the up slope 102b12, and from equation (4) their incidence angles are:θd2−3α<θc, θd3−3α>θc and θd4−3α>θc.Only the incidence angle of the light flux φ2, θd1−3α, is less than the critical angle θc and the flux φ2 with a width b2 exits from the up slope 102b12. Similarly, a fourth striking on the boundary surface results in the light flux φ3 with a width b3 exiting from the top surface 102a. On a fifth striking, the flux φ4 exits from the up slope 102b12 in a width b4.
As described above, these fluxes successively escape outside according to the number of times the light has struck the boundary surfaces. The width of the light flux as it leaves the light guide plate 102 progressively increases according to the number of boundary surface striking times. The light flux widths has the following relation:b1<b2<b3<b4.
This may be explained as follows. Although the fluxes φ1, φ2, φ3, φ4 have the same angular range, the actual width of each flux increases in proportion to the length of flux path. The larger the number of times the flux strikes the boundary surfaces, the longer the flux path and the larger the actual width of the flux. In FIG. 10, the flux that has escaped from the up slope 102b12 of the bottom surface is reflected by the reflector plate 106 as shown in FIG. 9 and reenters the light guide plate 102 before exiting the top surface 102a. It is thus considered possible to deal with this light flux in the same way as if it first exited from the top surface 102a. 
In the following discussion, the outgoing light rays of the fluxes φ1, φ2, φ3, φ4 of FIG. 10 are considered in relation to the bright lines 14{circle around (1)}, 14{circle around (2)}, 14{circle around (3)}, 14{circle around (4)} shown in FIG. 8. An exit light of the flux φ1 with the width b1 shown in FIG. 10 is situated closest to the light receiving side surface 102c and its width is narrowest, so that it is considered to correspond to the bright line 14{circle around (1)} of FIG. 8. An exit light of the flux φ2 with the width b2 shown in FIG. 10 is situated second closest to the light receiving side surface 102c and its width is slightly wider than b1. So, this flux is considered to match the bright line 14{circle around (2)} of FIG. 8. Similarly, exit light of the fluxes φ3, φ4 shown in FIG. 10 is considered to correspond to the bright lines 14{circle around (3)}, 14{circle around (4)}, respectively.
If the refracted angle of the flux φ4 on the top surface 102a shown in FIG. 10 increases, the number of times that the flux strikes the boundary surface before it exits the light guide plate 102 also increases and the position at which it leaves the light guide plate 102 is further away from the light receiving side surface 102c, increasing the width of the outgoing flux, which in turn is considered to reduce a light quantity per unit area or brightness. This agrees with the fact that, as the bright lines shown hatched with thick lines in the FIG. 8 plan view of the light guide plate 102 move away from the light receiving side surface 102c, the width of the bright lines increases, making them less conspicuous. Therefore, the marked bright lines such as shown in FIG. 8 are considered to be caused by the presence of those light rays emitted from the edge portion 102d which go out of the light guide plate 102 with a relatively small number of boundary surface striking times (1–4 times).
In the above description the edge portion 102d of FIG. 9 has been described to be a secondary light source. In a case where the lower edge portion 102e functions as a secondary light source, too, light rays with their incidence angles on the top surface 102a greater than the critical angle produce bright lines according to the same principle described above. These bright lines tend to become more conspicuous when a member for restricting a direction of light, such as the prism sheet 103 of FIG. 6 or diffusion sheet (not shown), is installed above the light guide plate 102. Although the above description has taken up, as an example case of bright lines, a construction in which a reflection surface of the bottom surface 102b of the light guide plate 102 is formed of asymmetric prisms 102b1, the principle of bright line generation also applies to other constructions in which the reflection surface on the bottom surface 102b of the light guide plate 102 is formed of symmetric prisms or formed with a regular or irregular pattern of recessed and raised portions formed by printing, surface texturing or dot formation. That is, even in these cases, as long as a secondary light source is produced at the edge portions 102d, 102e of the light guide plate 102, there is a problem that bright lines show up when the light guide plate 102 is illuminated. When these bright lines are produced, bright and dark fringes show up, marring the appearance of the planar light source.